Preparation method of flat plate microfiltration membrane with mirabilite lattice structure

Abstract

This invention belongs to the field of water treatment technology, specifically relating to a method for preparing a flat sheet microfiltration membrane with a Glauber's salt lattice structure. The invention includes base membrane preparation, functional group introduction, and micropore formation. First, polypropylene is selected as the support fabric, and a polysulfone solution prepared by dissolving polysulfone in chloroform is coated onto the polypropylene support fabric to obtain a homogeneous base membrane. Then, a mixed solvent of n-butanol and sulfolane is selected as the surface treatment solvent to introduce negatively charged functional groups. Finally, the negatively charged flat sheet microfiltration membrane is fabricated by drilling holes using a photolithography machine with a wavelength of 540-550 nm blue light as the light source, a precision of 50 nm, and a power of 20-30 W. The advantages of this invention are that it achieves stable and high membrane flux and low effluent turbidity during water treatment.

Description

Technical Field

[0001] This invention pertains to the field of material preparation methods in the field of water treatment technology, specifically relating to a method for preparing a flat microfiltration membrane with a Glauber's salt lattice structure. Background Technology

[0002] The history of microfiltration membranes can be traced back to the early 20th century. In 1906, Beehhold published the first report on the properties of microfiltration membranes, proposing a method to change the membrane pore size by altering the polymer concentration. In 1918, Zsigmondy and Bachmann, utilizing the research results of their predecessors, developed production technologies for preparing nitrocellulose and cellulose acetate membranes. In 1925, the world's first microfiltration membrane company was established in Göttingen, Germany, specializing in the production and sale of microfiltration membranes. In 1927, Sartorius-Werke GmbH in Germany improved Zsigmondy's process technology and began small-scale commercial production of membrane filters. In 1960, Loeb and Sourirajan successfully developed asymmetric reverse osmosis cellulose acetate (CA). From the 1960s onwards, with the development of polymer materials, the study of membrane formation mechanisms, and the advancement of membrane fabrication technologies, membrane technology entered a stage of rapid development. Membrane materials have expanded from the initial asymmetric cellulose acetate (CA) membranes to include polysulfone (PSF), polyacrylonitrile (PAN), polyethersulfone (PES), polyvinylidene fluoride (PVDF), and various synthetic polymer membranes. Membrane modules have evolved from the initial single membrane sheets to plate membranes, tubular membranes, and hollow fiber membranes, with a series of membrane pore sizes. Microfiltration has rapidly developed from a laboratory-scale separation method into an important industrial unit operation technology, and it is increasingly widely used in food, pharmaceuticals, electronics, water treatment, and emerging biotechnology fields.

[0003] Regarding the mechanism of microfiltration, it was initially believed that the microfiltration process was essentially a simple mechanical sieving process. However, years of research have revealed that, in addition to mechanical sieving, two other factors determine the separation characteristics of the membrane: first, the interaction between the solute, solvent, and membrane material. These forces include van der Waals forces, electrostatic forces, and hydrogen bonding. The attraction or repulsion of solute molecules on the membrane surface or pore walls affects the membrane's separation efficiency; second, the average pore size and pore size distribution of the membrane influence its separation characteristics. After extensive experiments and electron microscopy observations, scholars such as Ye Lingbi concluded that the retention effects of microfiltration membranes can be broadly categorized into four types: ① mechanical retention; ② adsorption retention; ③ bridging retention; and ④ retention within the network of network-type membranes.

[0004] In addition, microfiltration generally operates in two modes: dead-end filtration and cross-flow filtration. For raw water with low turbidity and good quality, dead-end filtration is typically used, which significantly reduces energy consumption. For water with high turbidity and severe pollution, cross-flow filtration is used to prevent the accumulation of large amounts of contaminants on the membrane surface, thus avoiding membrane fouling and reduced filtration performance. Cross-flow filtration, also known as dynamic filtration, is superior because during microfiltration, the membrane surface is continuously subjected to the scouring and shearing action of the flowing solution, promptly washing away large molecular solutes accumulated on the membrane surface, preventing the formation of a concentration polarization layer, mitigating membrane fouling, and reducing the frequency of membrane cleaning.

[0005] When microfiltration membrane technology is used to remove algae and turbidity from water, the removal efficiency is related to the membrane pore size. Algae are generally larger than 1 micrometer in diameter, which is larger than the microfiltration membrane pore size. Therefore, microfiltration membranes are very effective at removing both cyanobacteria and green algae, achieving a near 100% algae removal rate in the filtered water. Turbidity is caused by suspended solids, which include water-insoluble inorganic and organic matter, as well as silt, clay, and microorganisms. These substances have particle sizes of approximately 10 μm to 4 mm, larger than the microfiltration membrane pore size. Therefore, microfiltration membranes are also very effective at removing turbidity, and the influent turbidity has no significant impact on the effluent turbidity. [4] Researchers have combined ozone pre-oxidation with microfiltration (MF) membrane technology to treat surface water containing Chlorella. They found that this combined approach effectively inactivates and retains the algae. Experiments demonstrated that the concentration of extracellular organic matter significantly impacts the resistance of the sludge cake layer during membrane filtration. Mo et al. used a 0.1 μm microfiltration membrane to treat slightly polluted raw water. Despite significant fluctuations in raw water quality, the effluent turbidity was <1 NTU, indicating a turbidity removal efficiency greater than 90%. Botes' operational experience in a water plant showed that ultrafiltration membranes can achieve a turbidity removal rate of over 95%, with stable effluent turbidity unaffected by influent turbidity.

[0006] The common feature of all the microfiltration membranes mentioned above is that they utilize the principle of sieving to retain pollutants by making the membrane pore size smaller than the diameter of the pollutants. However, the retention rate is very poor for some soft or small-particle pollutants. This invention fully analyzes some characteristics of pollutants and finds that most pollutants affecting turbidity are negatively charged. If the microfiltration membrane surface has a negative charge, the retention rate of soft or small-particle pollutants will be significantly improved. Combining this with the principle that the electric field force of a planar surface is greater than that of a spherical surface, we have developed a negatively charged planar microfiltration membrane to solve the above problems. Summary of the Invention

[0007] The main objective of this invention is to develop a negatively charged flat sheet microfiltration membrane that can solve the existing technical problems.

[0008] This invention is achieved through the following technical solution: A method for preparing a negatively charged flat-plate microfiltration membrane, characterized by comprising base membrane preparation, functional group introduction, and micropore formation; specifically: (1) Preparation of base film: Polypropylene is selected as the support cloth, and polysulfone is dissolved in chloroform to prepare a polysulfone solution; the polysulfone solution is scraped onto the polypropylene support cloth, and after scraping, it is immersed in anhydrous ethanol solution, taken out and air-dried, and then heat-treated to obtain a homogeneous base film. (2) Etching of the lattice structure: The base film in step (1) is treated for 2-5 hours with a mixture of n-butyl ketone and sulfolane as the surface treatment solvent at a temperature of 35-60°C. The mass percentage of n-butyl ketone in the surface treatment solvent is 20-60%. The ultrafine particles of nickel crystal with a particle size of 0.5-5 micrometers are used as templates. The templates are uniformly sprayed onto the surface of the base film and heated to 120-150°C. Then, under nitrogen protection at an ambient temperature of 50-60°C, the film is allowed to stand for 10-50 minutes and then cooled to room temperature. Finally, the film is washed off from the ultrafine particles of nickel crystal with pure water at a pressure of 0.1-0.3 MPa, thus completing the etching process of the lattice structure. (3) Micropore formation: Micropores are formed by photolithography. Blue light with a wavelength of 540-550nm is used as the light source. The photolithography machine with a precision of 50 nanometers and a power of 20-30W is used for processing. The negatively charged base film is punched to obtain the negatively charged flat sheet microfiltration membrane.

[0009] As a preferred embodiment, in the base membrane preparation process of the above-mentioned method for preparing a negatively charged flat plate microfiltration membrane, the thickness of the support cloth is selected to be 0.1-0.3 mm and the mesh size is 100-300 mesh; the coating thickness is controlled to be 0.05-0.5 mm.

[0010] Preferably, in the preparation process of the base membrane of the above-mentioned method for preparing a negatively charged flat plate microfiltration membrane, after the base membrane is coated, it is immersed in an anhydrous ethanol solution at a temperature of 20-35°C for 1-5 hours.

[0011] As a preferred embodiment, in the base membrane preparation process of the above-mentioned method for preparing a negatively charged flat plate microfiltration membrane, the membrane is removed and air-dried for 10-30 minutes, and then subjected to heat treatment at 70-80°C for 5-10 minutes.

[0012] Preferably, in the functional group introduction process of the above-mentioned method for preparing a negatively charged flat plate microfiltration membrane, surface treatment is performed for 2-5 hours at a temperature of 25-50℃.

[0013] Preferably, in the micropore formation process of the above-mentioned method for preparing a charged flat plate microfiltration membrane, the diameter of the processed circular pores is three types: 0.1 micrometer, 0.2 micrometer, or 0.45 micrometer, and the porosity is controlled at 30-50%.

[0014] Beneficial effects: The microporous membrane prepared by this invention can achieve stable membrane flux, large flux, and low turbidity of effluent in water treatment process. Detailed Implementation

[0015] Example 1 Polypropylene support fabric is coated with polysulfone to form a base film. A 0.1 mm thick polypropylene support film with a mesh size of 200 mesh is coated with a 30% polysulfone solution dissolved in chloroform. The coating thickness is controlled to be 0.1 mm. After coating, it is immersed in anhydrous ethanol solution at 30°C for 3 hours, air-dried for 20 minutes, and then heat-treated at 75°C for 10 minutes to obtain a homogeneous base film. A mixture of n-butanol and sulfolane was used as the surface treatment solvent for 3 hours at 35°C, with n-butanol accounting for 50% of the total mass. After treatment, the surface was immersed in chlorosulfonic acid for 1.5 hours to complete the introduction of negatively charged functional groups. Ultrafine nickel crystal particles with a particle size of 0.5-5 micrometers were used as a template agent, uniformly sprayed onto the substrate surface, and heated to 120-150°C. Then, under nitrogen protection at an ambient temperature of 50-60°C, the surface was allowed to stand for 10-50 minutes, cooled to room temperature, and finally rinsed off from the ultrafine nickel crystal particle substrate film with pure water at a pressure of 0.1-0.3 MPa, thus completing the etching process of the crystal structure. Using blue light with a wavelength of 545nm as the light source, a photolithography machine with a precision of 50 nanometers and a power of 30W is used to process the negatively charged base film to create holes with a diameter of 0.1 micrometers and a porosity controlled at 40%. Tests were conducted on wastewater with a turbidity of 50-100 NTU. At an operating pressure of 0.02 MPa, the membrane flux of the membrane in this embodiment can stably reach 100 liters / hour / square meter, and the effluent turbidity is less than 0.2 NTU.

[0016] Example 2 Polypropylene support fabric is coated with polysulfone to form a base film. A 0.2 mm thick polypropylene support film with a mesh size of 300 mesh is coated with a 40% polysulfone solution dissolved in chloroform. The coating thickness is controlled to be 0.1 mm. After coating, it is immersed in anhydrous ethanol solution at 35°C for 2 hours, air-dried for 30 minutes, and then heat-treated at 65°C for 20 minutes to obtain a homogeneous base film. A mixture of n-butanol and sulfolane was used as the surface treatment solvent for 3 hours at 35°C, with n-butanol accounting for 50% of the total mass. After treatment, the surface was immersed in chlorosulfonic acid for 2 hours to introduce negatively charged functional groups. Ultrafine nickel crystal particles with a particle size of 0.5-5 micrometers were used as a template agent, uniformly sprayed onto the substrate surface, and heated to 120-150°C. Then, under nitrogen protection at an ambient temperature of 50-60°C, the surface was allowed to stand for 10-50 minutes, cooled to room temperature, and finally rinsed off from the ultrafine nickel crystal particle substrate film with pure water at a pressure of 0.1-0.3 MPa, thus completing the etching process of the crystal structure. Using blue light with a wavelength of 545nm as the light source, a photolithography machine with a precision of 50 nanometers and a power of 30W is used to process the negatively charged base film to create holes with a diameter of 0.2 micrometers and a porosity controlled at 35%. Tests were conducted on wastewater with a turbidity of 50-100 NTU. At an operating pressure of 0.02 MPa, the membrane flux could stably reach 130 liters / hour / square meter, and the effluent turbidity was less than 0.1 NTU.

[0017] Example 3 Polypropylene support fabric is coated with polysulfone to form a base film. A 0.3 mm thick polypropylene support film with a mesh size of 300 mesh is coated with a 40% polysulfone solution dissolved in chloroform. The coating thickness is controlled to be 0.15 mm. After coating, it is immersed in anhydrous ethanol solution at 50°C for 2 hours, air-dried for 20 minutes, and then heat-treated at 60°C for 30 minutes to obtain a homogeneous base film. A mixture of n-butanol and sulfolane was used as the surface treatment solvent for 3 hours at 35°C, with n-butanol accounting for 30% of the total mass. After treatment, the surface was immersed in acrylic acid or chlorosulfonic acid for 3 hours to introduce negatively charged functional groups. Ultrafine nickel crystal particles with a particle size of 0.5-5 micrometers were used as a template agent, uniformly sprayed onto the substrate surface, and heated to 120-150°C. Then, under nitrogen protection at an ambient temperature of 50-60°C, the surface was allowed to stand for 10-50 minutes, cooled to room temperature, and finally rinsed off from the nickel crystal ultrafine particle substrate film with pure water at a pressure of 0.1-0.3 MPa, thus completing the etching process of the crystal structure. Using blue light with a wavelength of 550nm as the light source, a photolithography machine with a precision of 50nm and a power of 30W is used to process the negatively charged base film to create holes with a diameter of 0.45 micrometers and a porosity of 45%. Tests were conducted on wastewater with a turbidity of 50-100 NTU. At an operating pressure of 0.02 MPa, the membrane flux could stably reach 180 liters / hour / square meter, and the effluent turbidity was less than 0.2 NTU.

[0018] Example 4 The base film and post-treatment are the same as in Example 3. Using blue light with a wavelength of 540nm as the light source, a photolithography machine with a precision of 50nm and a power of 25W is used to process the negatively charged base film to create holes with a diameter of 0.2 micrometers and a porosity controlled at 50%. Tests were conducted on wastewater with a turbidity of 50-100 NTU. At an operating pressure of 0.02 MPa, the membrane flux could stably reach 150 liters / hour / square meter, and the effluent turbidity was less than 0.2 NTU.

[0019] Implement comparative ratio 5-9 Commercially available membranes with pore sizes of 0.45, 0.2, and 0.1 micrometers, made of polypropylene, polyethylene, polysulfone, polytetrafluoroethylene, polyvinylidene fluoride, and membranes from several embodiments of this invention, were used to conduct comparative tests on wastewater with an influent turbidity of 50-100 NTU at an operating pressure of 0.02 MPa. The comparative data are shown in the table below.

[0020] Example 1 Sample Polypropylene + Polysulfone 0.1 100 0.2 Example 2 Sample Polypropylene + Polysulfone 0.2 130 0.1 Example 3 Sample Polypropylene + Polysulfone 0.45 180 0.2 Example 4 Sample Polypropylene + Polysulfone 0.2 150 0.2 Example 5 Product Film 1# polypropylene 0.45 80 3 Example 6 Product Film 2# polysulfone 0.1 50 0.3 Example 7 Product Film #3 polytetrafluoroethylene 0.45 No water comes out No water comes out Example 8 Product Film #4 Polyvinylidene ethylene 0.2 50 0.8 Example 9 Product Film 5# Polyvinylidene ethylene 0.1 10 0.6

[0021] As can be seen from the above list, the membrane prepared according to the method of this application has significantly superior performance in terms of water flux, water turbidity, and other comprehensive properties.

Claims

1. A method for preparing a flat microfiltration membrane with a Glauber's salt lattice structure, characterized in that... Includes the following steps: (1) Preparation of base film A base film is formed by blending chlorinated polypropylene, chlorinated polyethylene, and polystyrene propylene. The molecular weight of the chlorinated polypropylene granules is 2-5 million, the molecular weight of the chlorinated polyethylene granules is 5-8 million, and the molecular weight of the polystyrene propylene is 1-2 million. The blends are carried out in a weight ratio of 1-3:5-8:1-5 and then stretched into a base film with a thickness of 0.1-0.4 mm at a temperature of 120-170℃. (2) Etching of crystal lattice structure Using a mixture of n-butyl ketone and sulfolane as the surface treatment solvent, the base film in step (1) is treated for 2-5 hours at a temperature of 35-60°C; the mass percentage of n-butyl ketone in the surface treatment solvent is 20-60%; using ultrafine nickel crystal particles with a particle size of 0.5-5 micrometers as templates, the templates are uniformly sprayed onto the surface of the base film and heated to 120-150°C; then, under nitrogen protection at an ambient temperature of 50-60°C, the film is allowed to stand for 10-50 minutes and then cooled to room temperature, and then washed off from the ultrafine nickel crystal particle base film by reverse rinsing with pure water at a pressure of 0.1-0.3 MPa, thereby completing the etching process of the crystal structure. (3) Formation of micropores Micropores are fabricated using a photolithography machine. Blue light with a wavelength of 540-550nm is used as the light source. The photolithography machine with a precision of 50 nanometers and a power of 50-100W is used to process and punch holes in the base film in step (2), thereby obtaining a flat microfiltration membrane with a Glauber's salt lattice structure.

2. The preparation method according to claim 1, characterized in that: The base film treated with the surface treatment solvent in step (2) is then soaked in acrylic acid or chlorosulfonic acid for 0.5-3 hours to complete the surface energy reduction process.

3. The preparation method according to claim 1, characterized in that: In step (2), when the template agent is sprayed onto the surface of the base film, the spraying side of the base film is facing upwards, and then the metallic nickel is heated by microwave; the base film that is in contact with the ultrafine particles of metallic nickel crystals begins to melt, and the particles sink downwards under the action of gravity, thus forming a sunken shape.

4. The preparation method according to claim 1, characterized in that: In step (3), when drilling holes in the base film with the etched lattice structure, the diameter of the holes is one of three sizes: 0.1 micrometers, 0.2 micrometers, or 0.45 micrometers, and the porosity is controlled between 30-50%.